As power supplies for driving portable electronic equipment such as cell phones, portable personal computers, and portable music players, sealed batteries such as alkaline secondary batteries represented by a nickel hydrogen battery and nonaqueous electrolyte secondary batteries represented by a lithium ion battery are widely used.
As shown in FIG. 5, a related-art sealed battery 50 commonly includes an outer can 1 having electric power generating elements such as an electrode assembly, a sealing plate 2 for sealing an upper mouth portion of the outer can 1, and two external electrode terminals 3a and 3b each protruding from each side of the sealing plate 2. Furthermore, the sealing plate 2 has a gas exhaust valve 4 for releasing internal pressure when the pressure in the outer can 1 is increased, along with an electrolyte pour hole 5 for pouring an electrolyte into the outer can 1.
FIG. 5 does not directly show the electrolyte pour hole 5 and only shows a flange part of a blind rivet 6 (hereinafter, simply referred to as “rivet”) for sealing the electrolyte pour hole 5. In this manner, the electrolyte pour hole 5 is sealed with the rivet 6 so that the poured electrolyte would not leak from the outer can 1 (see, for example, JP-U-59-44027 and JP-A-2003-229118).
FIG. 6 show a sealing structure of the electrolyte pour hole with the rivet in such a related-art sealed battery. Here, FIG. 6A is a plan view of a related-art sealed battery 50, FIG. 6B is a cross-sectional view showing the vicinity of the electrolyte pour hole taken along the line VIB-VIB of FIG. 6 A, and FIG. 6C is a cross-sectional view showing the part corresponding to that in FIG. 6B before the rivet that is attached to the pour hole is crimped. On a peripheral surface of the electrolyte pour hole 5, an annular convex part 7 is formed so as to surround the electrolyte pour hole 5 and to protrude in the axis direction of the can.
As shown in FIG. 6B, the rivet 6 is made of aluminum, has a shank part 6a inserted into the electrolyte pour hole 5, a flange part 6b covering the peripheral surface of the electrolyte pour hole 5, and a crimping part 6c, and is crimped to be fixed to the sealing plate 2 interposing an annular gasket 8 between the flange part 6b and the sealing plate 2. Thus, the annular gasket 8 is interposed between the electrolyte pour hole 5 and the rivet 6. An inner peripheral part 8a of the gasket 8 is partially strongly compressed by the annular convex part 7 and the flange part 6b of the rivet 6, and thus the electrolyte pour hole 5 maintains a high sealing performance.
As disclosed in, for example, JP-A-2003-229118, the crimping part 6c of the rivet 6 is formed in the following manner. That is, as shown in FIG. 6C, a rivet 6 is prepared. The rivet 6 includes in its inside a stainless steel center shank part 6f having an enlarged diameter part 6d at the leading end and having a reduced diameter part 6e on the upper part of the enlarged diameter part 6d and includes a flange part 6b. The rivet 6 further includes a cylinder-shaped shank part 6a that is inserted into an electrolyte pour hole 5. The shank part 6a has an envelope-shaped leading end. The annular gasket 8 is fitted to an outer periphery of the shank part 6a of the rivet 6, and then the shank part 6a of the rivet 6 is inserted into the electrolyte pour hole 5 so that the annular gasket 8 would be placed between the flange part 6b and the sealing plate 2.
Next, when the center shank part 6f is pulled upward with the flange part 6b of the rivet 6 pressed toward the sealing plate 2, the enlarged diameter part 6d at the leading end of the center shank part 6f is moved upward. Consequently, the diameter of the envelope-shaped leading end of the shank part 6a of the rivet 6 is enlarged to form a crimping part 6c. Thus, the rivet 6 is fixed in the electrolyte pour hole 5 and the center shank part 6f of the rivet 6 is cut off at the reduced diameter part 6e formed on the upper part of the enlarged diameter part 6d. As a result, the electrolyte pour hole 5 can be sealed fluid-tightly with the rivet 6. In FIG. 6B, actually, a void is formed in the rivet 6 and the enlarged diameter part 6d of the center shank part 6f stays in the rivet 6, but they are not shown in the drawing.
As described above, when the annular convex part 7 is formed on the peripheral surface of the electrolyte pour hole 5, the annular convex part 7 and the inner peripheral part 8a of the gasket 8 are partially strongly compressed by the annular convex part 7 and the flange part 6b of the rivet 6, so that the electrolyte pour hole 5 has a high sealing performance. However, as shown in FIG. 6B, the outer peripheral part 8b of the gasket that is not partially compressed by the annular convex part 7 bends downward, and consequently, only the side edge is sometimes brought into contact with the sealing plate 2. As a result, between the outer peripheral part 8b of the gasket and the surface of the sealing plate 2, an enclosed space S is formed.
Generally, in a pour process of an electrolyte, because an electrolyte adheres to and remains on the peripheral surface of the electrolyte pour hole 5, washing is performed after sealing the electrolyte pour hole 5 in order to remove the adhered electrolyte. However, when the electrolyte remains in the enclosed space S, the electrolyte cannot be removed by the washing. Moreover, the electrolyte remaining in the enclosed space S after washing is gradually leached out from the gasket 8 after the washing process, then the outside of the gasket 8 changes in color, and as a result, the sealed battery 10 has a poor appearance.
In order to solve the above problems, the inventors of the present invention have found that, for example as shown in FIG. 7A, when the annular convex part 7 is placed close to the outer peripheral part 8b of the gasket 8, the gasket 8 can be compressed near the outer peripheral part 8b of the gasket 8 by the annular convex part 7 and the rivet 6, and therefore the remaining electrolyte can be suppressed from leaching out from the enclosed space S. However, it is found that a battery having such a structure has other problems. That is, when the rivet 6 is crimped for fixing, a large upward force is applied to the peripheral part of the electrolyte pour hole 5. Thus, among the peripheral part of the electrolyte pour hole 5, a part P that is placed inside the annular convex part 7 is tend to be deformed when the rivet 6 is crimped for fixing, and consequently the adhesion between the gasket 8 and the annular convex part 7 becomes poor.
Hence, as shown in FIG. 7B, an annular convex part 7 having a wider width than that in the related art is prepared. Such a structure can prevent the enclosed space S from forming as well as can maintain a high mechanical strength of the part P because the part P has a heavy wall thickness. Moreover, because of a short distance between the flange part 6b of the rivet 6 that is pressed downward when crimping and the sealing plate 2 that is pressed upward when crimping, deformation of the part P when crimping is inhibited. However, it is found that a battery having such a structure has other problems. That is, a larger contact area between the surface of the annular convex part 7 and the gasket 8 also causes poor adhesion therebetween.